A steam generator is a closed, heated vessel or a pressure tube system, which serves the purpose of generating steam of high pressure and high temperature for heating and operating purposes (e.g. to operate a steam turbine). Where there are particularly high steam outputs and pressures, as for example when generating energy in power plants, water tube boilers are used, in which the flow medium—usually water—is present in steam generator tubes. Water tube boilers are also used for the combustion of solid matter, as the combustion chamber in which heat is generated by the combustion of the respective solid matter can be embodied as required by the arrangement of tube walls.
Such a steam generator with the structure of a water tube boiler therefore comprises a combustion chamber, the enclosing wall of which is formed at least partially from tube walls, in other words steam generator tubes welded in a gas-tight manner. On the flow medium side these steam generator tubes first act in the manner of evaporator heating surfaces to form an evaporator, in which unevaporated medium is introduced and evaporated. The evaporator here is usually disposed in the hottest region of the combustion chamber. Connected downstream of it on the flow medium side, if required, are a facility for separating water and steam and a superheater, in which the steam is heated further beyond its evaporation temperature in order to achieve a high degree of efficiency in a subsequent heat engine, as for example during expansion in a steam turbine. A preheater (or economizer) can be connected upstream of the evaporator on the flow medium side, to preheat the supply water using waste heat or residual heat and thus also to increase the efficiency of the plant as a whole.
Depending on the structure and geometry of the steam generator, further steam generator tubes can be disposed within the combustion chamber, being connected parallel to the steam generator tubes forming the enclosing walls on the flow medium side. They can be joined or welded together for example to form an inner wall. Depending on the desired arrangement of evaporator heating surfaces or inner walls within the combustion chamber, it may be necessary to interleave inner walls one behind the other on the flow medium side and to connect their steam generator tubes by way of an intermediate collector.
This is the case for example with the so-called pant-leg design for steam generators with fluidized bed combustion. With these two inner walls formed at least partially from further steam generator tubes and disposed symmetrically in the combustion chamber are connected upstream of an intermediate collector on the flow medium side. The medium flow from the upstream inner wall combines in the intermediate collector and it serves as an inlet collector for a downstream inner wall. The pant-leg design provides better mixing of the fuel mixture and therefore fewer possible distribution problems on the combustion side.
However in certain operating states a steam content greater than zero can already result in the intermediate collector. Such a steam content renders impossible a regular distribution of the medium to the downstream inner wall with a simple collector, so that water/steam separation can occur. Individual tubes of the downstream inner wall can thus have such high steam contents or enthalpies at their inlet that superheating of said tubes is very likely. Such superheating can cause tube damage during longer-term operation.